There are two main types of avalanche: loose snow avalanches and slab avalanches. Both kinds can involve either wet or dry snow and may involve parts of the snow cover, or all of it down to the bare ground. A loose snow avalanche is characterised by starting at a particular point and then spreading out progressively in a pear-shaped course. At the release point the gradient must be at least 35°. A slab avalanche has a distinct line of cleavage through the snow at right angles to the surface, and it achieves its full width of front immediately. A slab avalanche can occur on any slope above 25°.
Avalanche prediction is, at best, an inexact science. So many variables affect snow on different slopes under constantly changing weather conditions that it is very difficult to assess or predict avalanches accurately. High avalanche danger results from storms with heavy snowfalls, when a fresh snow fall does not bond with the underlying layer, which may have been coarse or icy. The unstable layer does not have to be the top or most recent layer. A very thin layer a metre below the surface, with little bonding to layers above and below, could be triggered into an avalanche by the weight of a skier. Slopes to the lee and cornices tend to be particularly unstable. Eighty per cent of avalanches occur during or just after storms. Most avalanches which trap people are triggered by those people.
One cubic metre of new snow weighs between 30 kg and 66 kg while a cubic metre of damp, coarse grain snow can weigh 400 kg to 600 kg. An area 20 m wide and 25 m long covered to 20 cm contains 100 m3 of snow, therefore this cover weighs between one half and fifty tonnes!
Although avalanches are not all that common in Australia they have occurred, and there have been at least two deaths in recent years. In the northern hemisphere, only about 20% of those trapped by an avalanche are rescued alive by an organised search. About 20% of people caught in avalanches are killed more or less immediately by shock or mechanical injuries. After an hour, there is only a 40% chance of survival, and it diminishes by half for every half hour after that.
If you find yourself in an area of high avalanche risk, proceed with great caution. Make sure you can release your pack, poles and skis quickly, by undoing straps, safety bindings etc, as these will drag you down or injure you if caught in an avalanche. Try to avoid areas which show any signs of previous avalanche activity. Avoid wide, open snow areas, and any vigorous turning or movements which will increase the load on the snow cover. Leave the danger area by going up or down the fall line, on foot if necessary. Cross any suspected avalanche area as high up on the slope as possible. Groups should cross one at a time, keeping a distance of at least 50 m between skiers. Appoint someone to watch the slopes above while the rest make their way across.
If you are caught in an avalanche, try to move to the side of its downhill path, and head for rocks, trees or any other feature which may provide shelter. Move your arms in a swimming motion to try to stay on the surface of the snow. If buried, your best chance is probably to curl into a ball and protect your chest and respiratory passages with your arms in front of your body.
Skiers in powder snow and avalanche areas tie a 10–15 m length of red cord or cloth around their waist which would help in the event of a search party trying to locate them if buried in an avalanche. Another aid is a portable transceiver radio which sends out a continuous signal. Any other skier not swept away can set their radio to receive signals, and aid in locating the buried person.
As a fifth of those caught in avalanches are killed instantly, survival chances are not good, but a prompt search by companions may bring you out alive. Statistics show that most people surviving avalanche burial are rescued by their companions. An externally organised search takes time to arrange, and crucial time is lost getting help.
If searching for someone buried, start your search at the bottom edge of the avalanche debris, looking for skis, poles or other objects sticking out of the snow. Try to follow the likely path the person may have taken, searching around fixed objects such as trees and rocks in the avalanche path, or where the avalanche may have changed course. Remove the baskets from your poles, and use them as probes, working back up in the direction from which the avalanche came. Always be wary of further avalanches. Any detour, however long, must be worth the extra effort if it means not being caught in an avalanche and potentially losing companions, or killing yourself.